Ion exchange polymer membranes have found utility in a number of electrochemical and other processes. One use has been as membranes for solid polymer electrolyte cells. Solid polymer electrolyte cells typically employ a membrane of an ion exchange polymer which serves as a physical separator between the anode and cathode while also serving as an electrolyte. These cells can be operated as electrolytic cells for the production of electrochemical products or they may be operated as fuel cells for the production of electrical energy. Ion exchange polymer membranes are also used for facilitated transport, diffusion dialysis, electrodialysis, pervaporation and vapor permeation separations.
Membranes of highly fluorinated polymers such as perfluorinated sulfonic acid polymer membranes are particularly well-suited for such uses due to excellent chemical resistance, long life, and high conductivity. However, for some applications, the tensile strength of such membranes is not as high as desired and reinforcements are sometimes incorporated into the membranes to increase strength. For example, in membranes used in the chloralkali process, i.e., the production of caustic and chlorine by electrolytic conversion of an aqueous solution of an alkali metal chloride, woven reinforcements are incorporated into the membranes. While woven reinforcements work well in use, the fabrics are expensive and processes for incorporation of the fabrics into the membranes are cumbersome. For other applications such as in fuel cells, increased tensile strength is typically not needed in use but may be desirable for ease of handling or for certain manufacturing operations involving the membranes. Woven fabrics are generally unsuitable for membranes for fuel cells since membranes incorporating fabrics typically do not have the flat surfaces needed for contact with the electrodes employed in use in a fuel cell.
Composite ion exchange membranes have been developed which incorporate porous supports of a highly fluorinated nonionic polymer such as expanded polytetrafluoroethylene (EPTFE) to increase tensile strength and improve dimensional stability. However, the processes known for making such membranes are not particularly suitable for commercial manufacturing operations. For example, U.S. Pat. No. 5,082,472 dislocloses a process for making composite membrane intended for facilitated transport end use. In the process of this patent, the following steps are disclosed:
(1) Melt extrusion of the precursor of a perfluorinated ionomer to form a film; PA1 (2) Lamination of the precursor film to the EPTFE to form a precursor laminate; PA1 (3) Impregnation of the EPTFE component with a dilute (e.g., 2% solids) liquid composition of low equivalent weight ionomer followed by drying; and PA1 (4) Hydrolysis of the ionomer precursor film layer.
U.S. Pat. No. 5,082,472 teaches that the EPTFE side of the composite should preferably be coated with the liquid composition of the ionomer prior to hydrolysis. This patent explains that "[o]therwise, the ionomer film will swell, and the hydrophobic EPTFE will not allow the release of the hydrostatic pressure front the swelling, causing the structure to delaminate locally".
While the process of U.S. Pat. No. 5,082,472 can be used for making individual membranes, it is not easily adapted to larger scale manufacturing processes where processing speed is of high importance, e.g., continuous processes in which the ionomer film and the EPTFE are supplied as roll stock. It is difficult to fully impregnate the EPTFE film with the liquid ionomer composition because both the EPTFE and the ionomer film are hydrophobic. Impregnation times can be unacceptably long and/or voids may remain which adversely affect membrane properties.